1. Field of Invention
The present invention generally relates to an apparatus and method for electrical penetration of a nuclear detector tub, and in particular, electrical penetration along the axial or transaxial planes. Moreover, such electrical penetration may provide an integrated seal against visible light (light seal).
2. Description of Related Art
There are several distinctive types of imaging systems in contemporary nuclear medicine. One type may employ gamma scintillation cameras (GSCs), so-called “position sensitive” continuous-area detectors, or simply, nuclear detectors. An exemplary GSC is a single photon emission computed tomography (SPECT) scanner. Another type of imaging system involves computed tomography (“CT”) of X-ray imaging. By comparison, magnetic resonance imaging (MRI), formerly referred to as magnetic resonance tomography (MRT) or nuclear magnetic resonance (NMR), is a method used to visualize the inside of living organisms relying on the relaxation properties of excited hydrogen nuclei in water placed in a powerful, uniform magnetic field.
Insofar as each imaging system has it own advantages and disadvantages, clinics may have several different systems, possibly in proximity to each other. The closer the proximity of the systems, the greater the potential that exists that one system may interfere with another. The likelihood of a system interfering with another system depends in large part on the types of imaging technology involved and the sensitivity of the interfered-with system to the imaging technology of the interfering system. For instance, a CT scanner uses an active detection technology involving high-energy X-rays that easily may interfere with a GSC, which uses a passive-detection technology sensitive to low energy densities. While measures are taken, therefore, to effectively shield a GSC from interference from a CT scanner, these measures themselves may raise new challenges.
X-rays are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.1 nanometers, corresponding to frequencies in the range 30 to 3000 PHz (1015 hertz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. Electromagnetic waves with a wavelength approximately longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays.
The basic production of X-rays is by accelerating electrons in order to collide with a metal target (copper, molybdenum or tungsten usually). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This causes the spectral line part of the wavelength distribution. There is also a continuum bremsstrahlung component given off by the electrons as they are scattered by the strong electric field near the high Z (proton number) nuclei.
Computed tomography (CT), originally known as computed axial tomography (CAT or CAT scan) and body section roentgenography, is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word “tomography” is derived from the Greek tomos (slice) and graphia (describing). Computed tomography systems generate an infinite set of X-ray beam projections through an object to be examined.
CT systems subject the object under examination to one or more pencil-like X-ray beams from many directions. The X-ray beams passing through the object are attenuated by various amounts, depending upon the nature of the object traversed (e.g., bone, tissue, metal, etc.). One or more X-ray detectors, disposed on the far side of the object, receive these beams and provide analog output signals proportional to the strength of the incoming X-rays. Each detector output is then digitized and computer processed to help produce an image of a slice of the object.
The resultant detected X-ray data are computer processed to reconstruct a tomographic image-slice of the object. CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the x-ray beam. Although historically the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3 D) representations of structures.
Hard X-rays overlap the range of “long”-wavelength (lower energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei. It is important to note that there is no physical difference between gamma rays and X-rays of the same energy—they are two names for the same electromagnetic radiation. Rather, gamma rays are distinguished from X-rays by their origin.
Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation produced by radioactive decay or other nuclear or subatomic processes such as electron-positron annihilation. Gamma rays form the highest-energy end of the electromagnetic spectrum. They are often defined to begin at an energy of 10 keV, a frequency of 2.42 EHz, or a wavelength of 124 pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard X-rays.
Gamma scintillation cameras, GSCs, are primarily used to measure gamma events produced by very low-level radioactive materials (called radionuclides or radio-pharmaceuticals) that have been ingested by, or injected into, a patient. The signals from the GSCs are used to generate images of the anatomy of organs, bones or tissues of the body and/or to determine whether an organ is functioning properly. The radiopharmaceuticals are specially formulated to collect temporarily in a certain part of the body to be studied, such as the patient's heart or brain. Once the radio-pharmaceuticals reach the intended organ, they emit gamma rays that are then detected and measured by the GSCs. Nuclear detectors perform spectroscopy and event X/Y positioning by processing signals from a constellation of Photo-multiplier Tubes (PMTs). The current HD series detectors contain 59 PMTs.
While it may be convenient for a clinic to have a GSC system in proximity to a CT system, a significant problem may arise when this is done. In particular, if any X-rays of significant magnitude from the CT system infiltrate the GSC, the output of the GSC at best will be skewed. Indeed, a GSC includes a large area scintillation crystal, which functions as a gamma ray detector and is typically sodium iodide doped with a trace of thallium (NaI(Tl)). The crystal converts high-energy photons (e.g.; gamma rays and X-rays) into lower energy photons, i.e., visible light. The relatively high, and constant, energy profile of X-ray events (as compared with gamma ray events) will likely drive the crystal significantly more than would gamma rays, thereby skewing the detection function of the GSC. More likely, the relatively sensitive photochemistry of the scintillation crystal will be over driven by the X-rays and may take a very long time (sometimes hours) to settle and again become useful in measuring gamma rays. At worst, the crystal may be permanently damaged from excessive levels of X-ray radiation.
To guard against the deleterious effects of stray X-rays blinding the scintillation crystal and the PMTs and possibly frying the associated electronics, a GSC has a lead enclosure, i.e., tub, to block the stray X-rays. Furthermore, because the scintillation crystal emits faint amounts of light upon scintillation, the interior of the GSC must be shielded from ambient light that would blind the photosensors in the GSC used to measure the scintillation light emissions.
Due to the large number of interconnections between PMT preamplifiers and an acquisition electronics system, portions of the acquisition electronics system have been packaged inside the tub. The cables that historically did exit the tub from the interior portion of the acquisition electronics system required light seals, for which a compliant packable material was used, making the seals difficult to handle, not integrated with the tub, and non-reusable. Because these connections generate heat, a significant amount of heat accumulates in the tub, detrimentally affecting the GSC's reliability. Likewise, the connections involve numerous printed circuit boards (PCBs) and cables, which must be disassembled to access a PMT needing replacement, making the GSC hard to service. Similarly, because the detectors need to be built in test stands, disassembled and then reassembled in the tubs, this arrangement of the connections compounds the effort needed to manufacture the GSCs.
While this arrangement has been functional, reliability, serviceability and manufacturability would all benefit if the electronics could be relocated outside of the tub. For these electronics to be placed outside the tub, an electrical penetration of tub must support all of the PMT interconnections, including High Voltage PMT & dynode bias, and provide a light seal. Moreover, an electrical penetration should facilitate easy assembly & disassembly, which are important for manufacturing and service.
Accordingly, there is a need in the art for new methods and apparatus for enabling relocation of the electronics outside of the tub of a CGS.